LaTeX does not load packages more than once. Loading the package graphics the first time, enables draft mode. Package pdfpages loads graphicx and graphicx loads graphics without options each, thus LaTeX will not complain. The important thing is that the superset of all options specified in all load requests must be present at the first loading of the package.

Other alternatives are given in Gonzalo Medina's answer:

Global option draft is seen by packages graphics and graphicx.

Calling \PassOptionsToPackagebefore the first loading of graphics or graphicx.

Key value setting

And yet another way is possible:

\usepackage[draft]{pdfpages}
\setkeys{Gin}{draft}

Package graphicx adds a key value interface for some of the options. draft can be set and unset later: